Liquid inks are widely used in a variety of printing processes, for example offset, intaglio, rotogravure, ink jet and electrophotographic printing. Many of the desired characteristics of the pigment dispersions used in the liquid inks are the same for each of the respective processes even though the final ink formulations may be substantially different. For example, the stability of the pigment dispersion both on the shelf and under shear conditions is an important consideration regardless of the final use of the liquid ink. The art continuously searches for more stable pigment dispersions to provide more flexibility in ink formulations which in turn yields better efficiency and waste reduction in the various printing processes.
In electrophotographic applications, which includes devices such as photocopiers, laser printers, facsimile machines and the like, liquid inks are referred to as liquid toners or developers. Generally, the electrophotographic process includes the steps of forming a latent electrostatic image on a charged photoconductor by exposing the photoconductor to radiation in an imagewise pattern, developing the image by contacting the photoconductor with a liquid developer, and finally transferring the image to a receptor. The final transfer step may be performed either directly or indirectly through an intermediate transport member. The developed image is usually subjected to heat and/or pressure to permanently fuse the image to the receptor.
In the field of electrographic printing, particularly electrophotographic printing, a variety of both liquid and dry developing compositions have been employed to develop the latent electrostatic images. Dry toner compositions suffer from a number of disadvantages. For example, dry toners are known to be difficult to control during the latent image development and transfer processes; this leads to toner scatter within the printer device and may create excessive amounts of dust and abrasive wear of the printer components. Some dry toner compositions must also be fixed by fusing at elevated temperature, which requires a large source of energy and may limit the choices of receptor materials to which the developed latent image may be transferred. Moreover, dry toners must be triboelectrically charged, which makes the printing process very sensitive to both the temperature and humidity of the ambient air and may result in printing delays due to charge equilibration. The limited particle size of the toner is another disadvantage of dry toners. If the particle size is small, the dry toner can become airborne and create a potential health hazard due to inhalation of the particles. On the other hand, the larger particle sizes make it difficult to obtain high resolution images.
Many of the disadvantages accompanying the use of dry toner compositions have been avoided by the use of liquid developers or toners. For example, liquid toners contain smaller particles than dry toners resulting in higher resolution images. In addition, liquid toners are not triboelectrically charged; therefore, they are much less sensitive to changes in ambient temperature and humidity. Since the toner particles in a liquid developer are contained within a fluid phase, toner scatter and dust accumulation do not occur within the printer. In addition, the particles being contained within a liquid matrix will not become airborne thus eliminating the risk of inhalation of the particles.
Liquid toners typically comprise an electrically insulating liquid which serves as a carrier for a dispersion of charged particles known as toner particles composed of a colorant and a polymeric binder. A charge control agent is often included as a component of the liquid developer in order to regulate the polarity and magnitude of the charge on the toner particles. Liquid toners can be categorized into two primary classes, for convenience, the two classes will be referred to as conventional liquid toners and organosol toners.
Of particular utility are the class of liquid toners which make use of self-stable organosols as polymeric binders to promote self-fixing of a developed latent image. U.S. Pat. Nos. 3,753,760; 3,900,412; 3,991,226; 4,476,210; 4,789,616; 4,728,983; 4,925,766; 4,946,753; 4,978,598 and 4,988,602 describe the composition and use of these types of organosols. Self-stable organosols are colloidal (0.1-1 micron diameter) particles of polymeric binder which are typically synthesized by nonaqueous dispersion polymerization in a low dielectric hydrocarbon solvent. These organosol particles are sterically-stabilized with respect to aggregation by the use of a physically-adsorbed or chemically-grafted soluble polymer. Details of the mechanism of such steric stabilization are provided in Napper, D. H., Polymeric Stabilization of Colloidal Dispersions, Academic Press, New York, N.Y., 1983. Procedures for effecting the synthesis of self-stable organosols are known to those skilled in the art and are described in Dispersion Polymerization in Organic Media, K. E. J. Barrett, ed., John Wiley: New York, N.Y., 1975. Although it is generally recognized that the solvation of the particle is critical in the formation of a dispersion, none of the foregoing references recognize the utility of a gel in forming a stable dispersion.
The most commonly used non-aqueous dispersion polymerization method is a free radical polymerization carried out when one or more ethylenically-unsaturated (typically acrylic) monomers, soluble in a hydrocarbon medium, are polymerized in the presence of a preformed amphipathic polymer. The preformed amphipathic polymer, commonly referred to as the stabilizer, is comprised of two distinct repeat units, one essentially insoluble in the hydrocarbon medium, the other freely soluble. When the polymerization proceeds to a fractional conversion of monomer corresponding to a critical molecular weight, the solubility limit is exceeded and the polymer precipitates from solution, forming a core particle. The amphipathic polymer then either adsorbs onto or covalently bonds to the core, which core continues to grow as a discrete particle. The particles continue to grow until monomer is depleted; the attached amphipathic polymer "shell"acts to sterically-stabilize the growing core particles with respect to aggregation. The resulting core/shell polymer particles comprise a self-stable, nonaqueous colloidal dispersion (organosol) comprised of distinct spherical particles in the size (diameter) range 0.1-0.5 microns.
The resulting organosols can be subsequently converted to liquid toners by simple incorporation of the colorant (pigment) and charge director, followed by high shear homogenization, ball-milling, attritor milling, high energy bead (sand) milling or other means known in the art for effecting particle size reduction in a dispersion. The input of mechanical energy to the dispersion during milling acts to break down aggregated pigment particles into primary particles (0.05-1.0 micron diameter) and to "shred" the organosol into fragments which adhere to the newly-created pigment surface, thereby acting to sterically-stabilize the pigment particles with respect to aggregation. The charge director may physically or chemically adsorb onto the pigment, the organosol or both. The result is a sterically-stabilized, charged, nonaqueous pigment dispersion in the size range 0.1-2.0 microns, with typical toner particle diameters between 0.1-0.5 microns. Such a sterically-stabilized dispersion is ideally suited for use in high resolution printing.
Rapid self-fixing is a critical requirement for liquid toner performance to avoid printing defects (such a smearing or trailing-edge tailing) and incomplete transfer in high speed printing. A description of these types of defects and methods of preventing them using film forming compositions are described in U.S. Pat. Nos. 5,302,482; 5,061,583; and 4,925,766.
Another important consideration in formulating a liquid toner is the tack of the image on the final receptor. If the image has a residual tack, then the image may become embossed or picked off when placed in contact with another surface. This is especially a problem when printed sheets are placed in a stack. If the image is tacky, it may transfer to the backside of the adjacent sheet. To address this concern, a film laminate or protective layer is typically placed over the surface of the image. This adds both extra cost of materials and extra process steps to apply the protective layer.
It is further known in the art that film-forming liquid toners fabricated using self-stable organosols generally exhibit excellent aggregation stability; however, the sedimentation stability of such inks is poor. Once the components of an organosol ink have settled, they are generally difficult if not impossible to redisperse to a degree of dispersion equivalent to the original, unsettled ink. This situation arises because self-stable organosol inks settle into closely packed, dilatant sediments, and irreversible film formation can occur in these sediments when the volume fraction of organosol in the sediment exceeds the critical volume fraction required for film formation to occur (generally greater than 70 volume percent organosol). Hence, there is a need for liquid ink compositions which will overcome this poor sedimentation and redispersion behavior of organosol inks.
Attempts have been made to provide liquid developers having improved storage and thermal stability using a two component gel/latex system. U.S. Pat. Nos. 4,374,918; 4,363,863; 4,306,009; GB 2,066,493; and GB 2,065,320 describe liquid developers incorporating polymers having borderline solubility in the carrier solvent. A stabilizing gel and latex (or gelatex) are used as a dispersant and/or fixative. The gel and latex are separate non-covalently bonded materials in the ink formulation. Separate materials may adversely affect the aggregation stability which can lead to preferential depletion of one of the two components during extended printing thus adversely affecting print quality. Separate components can also result in a broad molecular weight distribution for stabilizing the gel, which may have an adverse affect on the toner charge characteristics. In addition, the separate materials may lead to a high free phase conductivity.
Attempts have also been made to overcome the poor sedimentation stability of pigmented liquid toners by replacing the pigment with a dye of significantly lower density. U.S. Pat. No. 4,816,370 describes a liquid developer using a thermally reversible, flocculated, dyed organosol. The colorant used is a dye rather than a pigment. It is well established in the art that dyes are less stable to light and have a tendency to migrate or sublime. Even though dyes have inherent advantages, such as transparency of the colors and less interference with the characteristics of the thermoplastic binders, their poor light stability often times overrides these advantages.
Currently, no one has sufficiently addressed the problem of sedimentation in pigmented liquid inks, in particular liquid toners.